Recently, the demand for high density recording has increased significantly in magnetic recording and reproducing devices, such as HDDs (Hard Disk Drives). The realization of high density recording is also necessary for magnetic heads and magnetic media to meet this demand. The magnetic recording and reproducing device includes a magnetoresistive head as a read sensor. This magnetoresistive head uses a structure called spin-valve, which employs the magnetoresistive effect of a multilayer film formed by laminating ferromagnetic metal layers with a nonmagnetic metal layer interposed therebetween. The magnetoresistive effect is a phenomenon in which the electrical resistance varies depending on the angle between the magnetizations of two ferromagnetic layers sandwiching a nonmagnetic intermediate layer. The spin-valve using the magnetoresistive effect has a structure of an anti-ferromagnetic layer/ferromagnetic layer/nonmagnetic intermediate layer/ferromagnetic layer. In this structure, the magnetization of the ferromagnetic layer contacting the anti-ferromagnetic layer is effectively fixed by an exchange coupling magnetic field generated in the interface between the anti-ferromagnetic layer and the ferromagnetic layer. At the same time, the magnetization of the other ferromagnetic layer is freely rotated by an external field, and thus an output is obtained. The ferromagnetic layer, whose magnetization is effectively fixed by the anti-ferromagnetic layer, is called pinned layer. The ferromagnetic layer, whose magnetization is rotated by the external field, is called free layer.
For the spin-valve employing the magnetoresistive effect, a CIP (Current In the Plane)-GMR (Giant Magneto-Resistive) head has been used to cause current to flow in the in-plane direction of the multilayer film. Today, CIP-GMR head is being replaced with TMR (Tunneling Magneto-Resistive) head and CPP (Current Perpendicular to the Plane)-GMR head to cause current to flow in the layer thickness direction of the multilayer film.
There are two major reasons for the replacement of the CIP-GMR head with the TRM head and CPP-GMR head. The first reason is that the TMR head and CPP-GMR head can increase the reproduction output more than the CIP-GMR head, thereby achieving high SNR (output/noise ratio). The second reason is that the CPP type causing the current to flow in the perpendicular direction of the multilayer film is more advantageous than the CIP type causing current to flow in the in-plane direction of the multilayer film, in terms of increasing the linear density. The linear density is the bit density in the circumferential direction of magnetic recording media. Incidentally, the bit density in the radius direction of the magnetic recording media is called the track density. The areal density of the magnetic recording and reproducing device increases by increasing both the linear density and the track density. The linear density can only be increased by increasing the resolution. The resolution indicates how high the reproduction output can be kept in high density recording, compared to in low density recording.
The existing magnetoresistive head has a configuration in which a magnetoresistive layer is sandwiched between a lower magnetic shield and an upper magnetic shield, which is a so-called shield-type read head. The resolution in the linear density direction is largely dependent on the magnetic shield gap (Gs). In other words, the smaller the magnetic shield gap is, the higher the resolution in the linear density direction is, and thus high areal density can be achieved. For the conventional CIP-GMR head, the magnetoresistive layer has had to be electrically isolated from the upper and lower magnetic shields, by interposing insulating layers between the upper and the lower magnetic shields, and the magnetoresistive layer, respectively. For this reason, it has been difficult to reduce the magnetic shield gap. On the other hand, for the TMR and CPP-GMR heads causing current to flow in the layer thickness direction of the multilayer film, there is no need to interpose the insulting layers between the upper and lower magnetic shields and the magnetoresistive layer, which is advantageous in reducing the magnetic shield gap. For this reason, the magnetoresistive head is shifting from the CIP-GMR head to the TMR and CPP-GMR heads, for increasing the output and resolution.
However, it is thought that it is difficult to reduce the layer thickness of the CPP type magnetoresistive layer to about 30 nm or less, and that the resolution increase will reach a limit in the near future. This is mainly due to the following two reasons. The first reason is that the layer thickness of the above magnetoresistive layer (anti-ferromagnetic layer/ferromagnetic layer/nonmagnetic intermediate layer/ferromagnetic layer) can be physically reduced to at most 30 nm. The second is that when the magnetic shield gap is about 30 nm or less, the media field applied to the read head rapidly decreases, and SNR rapidly decreases along with the reproduction output. When SNR decreases, the bit error rate (BER) does not increase even if a high resolution is obtained. The bit error rate, which is a bit signal error rate, indicates the total performance of the magnetic recording and reproducing device. In other words, it is difficult to achieve high areal density when the bit error rate is low. Due to these two reasons, the read head of the existing structure can only have a maximum magnetic shield gap of about 30 nm. This is a major impediment to increasing the areal density.
A so-called differential read head has been proposed as means for increasing the resolution in the linear density direction. In the longitudinal (in-plane) magnetic recording system, a signal field is generated only from the magnetization reversal area, with respect to the bit written in a magnetic recording media. While in a perpendicular magnetic recording system, a signal field is typically generated from each recorded bit. For this reason, the perpendicular magnetic recording system is suitable for the application of the differential read head. JP-A No. 183915/2002 discloses a read head structure in which two magnetoresistive layers are coupled in series via a conductive layer, thereby enabling to perform differential operation in a magnetic recoding and reproducing device using the perpendicular magnetic recording system. The pair of magnetoresistive layers is configured to have two free layers disposed adjacent to and facing each other via the conductive layer to serve as magnetic sensors for detecting signal fields, and to have the resistance change characteristics of opposite polarity to the magnetic field in one direction. Thus, the read head can perform differential operation. In this case, the resolution in the linear density direction is more influenced by the inside distance between the two free layers, than the magnetic shield gap. In other words, the resolution in the linear density direction is greatly influenced by the layer thickness of the conductive layer interposed between the pair of magnetoresistive layers. Thus, it is possible to obtain a high resolution in the liner density direction, by reducing the layer thickness of the conductive layer interposed between the pair of magnetoresistive layers, instead of reducing the magnetic shield gap. Further, JP-A No. 69109/2003 discloses a detailed structure of differential read head in which two free layers can have resistance change characteristics of opposite polarity to the magnetic field in one direction. Furthermore, JP-A No. 227749/2004 discloses a read head structure for achieving high resolution without providing the upper and lower magnetic shields.
In addition, the reproduction output of the differential read head is also thought to be increased. This is because if the maximum resistance change of one sensor is given by ΔR, the resistance change of all the read sensors is expected to be 2×ΔR.
In order to clarify whether the magnetic recording and reproducing device including a differential read head has a potential of high linear density, a study has been made on the reproduction characteristics by numerical computation using micromagnetic simulation and by measurement of bit error rate. The ratio (Gl/bl) of the distance (Gl) between a first free layer and a second free layer, to the bit length (bl) is set to 0.5. The linear density of the magnetic recoding and reproducing device is set to 2000 kfci. Here, the bit length is the recording bit length which is the physical length of perpendicular recording media.
As a result of this study, it has been found that in the magnetic recording and reproducing device including the differential read head, the resolution is increased more than with the existing head, but the reproduction output is reduced much more than with the existing head. Also, in the magnetic recording and reproducing device including the differential read head, there arises a problem that the bit error rate is more degraded than with the exiting head. This is because the reproduction output decreases and SNR decreases. Thus, it has been newly found that the areal density is unlikely to increase even if the differential read head is simply mounted on the magnetic recording and reproducing device.
The reason why the reproduction output of the differential read head decreases more than that of the existing shield-type head, is that the output of the shield-type head is maximum when the free layer is near the center of the recording pattern, while the output of the differential read head is maximum when the two free layers are in the transition area of the recording pattern. Here, the recording pattern is the area in which the magnetization of the perpendicular recording media is in one direction. The transition area of the recording pattern typically has a transition width. Thus, the media field applied to the free layer is reduced to about 20% to 40%, compared to when the free layer is located near the center of the recording pattern. For this reason, the reproduction output of the differential read head is lower than that of the existing head.